you have to contribute to the Canada Pension Plan (CPP) for 2018. This can apply if your total net self-employment income and pensionable employment income is more than $3,500. See line 222

you are paying employment insurance premiums on self-employment income or other eligible earnings. See lines 317 and 430

you have incurred a non-capital loss in the year that you want to be able to apply in other years. See line 236

you want to transfer or carry forward to a future year the unused part of your tuition fees. See line 323

you want to report income that would allow you to contribute to an RRSP, a pooled registered pension plan (PRPP), or a specified pension plan (SPP) to keep your maximum deduction limit (see Schedule 7) for future years up to date

you want to carry forward to a future year the unused investment tax credit on expenditures you incurred during the current year. See line 412